Historically, highly branched alkylbenzenesulfonate surfactants, such as those based on tetrapropylene (known as "ABS") were used in detergents. However, these were found to be very poorly biodegradable. A long period followed of improving manufacturing processes for alkylbenzenesulfonates, making them as linear as practically possible ("LAS"). The overwhelming part of a large art of linear alkylbenzenesulfonate surfactant manufacture is directed to this objective. All relevant large-scale commercial alkylbenzenesulfonate processes in use today are directed to linear alkylbenzenesulfonates. However, linear alkylbenzenesulfonates are not without limitations; for example, they would be more desirable if improved for hard water and/or cold water cleaning properties. Thus, they can often fail to produce good cleaning results, for example when formulated with nonphosphate builders and/or when used in hard water areas.
As a result of the limitations of the alkylbenzenesulfonates, consumer cleaning formulations have often needed to include a higher level of cosurfactants, builders, and other additives than would have been needed given a superior alkylbenzenesulfonate.
Accordingly it would be very desirable to simplify detergent formulations and deliver both better performance and better value to the consumer. Moreover, in view of the very large tonnages of alkylbenzenesulfonate surfactants and detergent formulations used worldwide, even modest improvements in performance of the basic alkylbenzenesulfonate detergent could carry great weight.
To understand the art of making and use of sulfonated alkylaromatic detergents, one should appreciate that it has gone through many stages and includes (a) the early manufacture of highly branched nonbiodegradable LAS (ABS); (b) the development of processes such as HF or AlCl.sub.3 catalyzed process (note each process gives a different composition, e.g., HF/olefin giving lower 2-phenyl or classic AlCl.sub.3 /chloroparaffin typically giving byproducts which though perhaps useful for solubility are undesirable for biodegradation); (c) the market switch to LAS in which a very high proportion of the alkyl is linear; (d) improvements, including so-called `high 2-phenyl` or DETAL processes (in fact not really "high" 2-phenyl owing to problems of solubility when the hydrophobe is too linear); and (e) recent improvements in the understanding of biodegradation.
The art of alkylbenzenesulfonate detergents is extraordinarily replete with references which teach both for and against almost every aspect of these compositions. For example, some of the art teaches toward high 2-phenyl LAS as desirable, while other art teaches in exactly the opposite direction. There are, moreover, many erroneous teachings and technical misconceptions about the mechanism of LAS operation under in-use conditions, particularly in the area of hardness tolerance. The large volume of such references debases the art as a whole and makes it difficult to select the useful teachings from the useless without large amounts of repeated experimentation. To further understand the state of the art, it should be appreciated that there has been not only a lack of clarity on which way to go to fix the unresolved problems of linear LAS, but also a range of misconceptions, not only in the understanding of biodegradation but also in basic mechanisms of operation of LAS in presence of hardness. According to the literature, and general practice, surfactants having alkali or alkaline earth salts that are relatively insoluble (their Na or Ca salts have relatively high Krafft temperature) are less desirable than those having alkali or alkaline earth salts which are relatively higher in solubility (Na or Ca salts have lower Krafft temperature). In the literature, LAS mixtures in the presence of free Ca or Mg hardness are said to precipitate. It is also known that the 2- or 3-phenyl or "terminal" isomers of LAS have higher Krafft temperatures than, say, 5- or 6-phenyl "internal" isomers. Therefore, it would be expected that changing an LAS composition to increase the 2- and 3-phenyl isomer content would decrease the hardness tolerance and solubility: not a good thing. On the other hand it is also known that with built conditions under which both the 2- and 3-phenyl and internal-phenyl isomers at equal chain length can be soluble, the 2- and 3-phenyl isomers are more surface-active materials. Therefore, it would be expected that changing an LAS composition to increase the 2- and 3-phenyl isomer content may increase the cleaning performance. However, the unsolved problems with solubility, hardness tolerance, and low temperature performance still remain.